This invention relates to methods for making investment shell molds for the high integrity, precision casting of reactive and refractory metals, and to the molds prepared by such methods.
In one method of making inventment shell molds (Brown et al U.S. Pat. No. 3422880), a disposable pattern is dip coated in a liquid suspension of columbium, molybdenum, tantalum or tungsten powder mold material and a metal oxide binder. The dip coated pattern is stuccoed with at least one of these metal powders, after which it is dried.
Additional dip coats and stucco coats of predetermined composition are applied in number sufficient to build up a finished mold of the desired composition and strength. The pattern is removed and the mold fired. It then is ready to receive molten titanium or other reactive and refractory casting metals.
The foregoing method of making a shell mold is highly useful and has the important advantage of producing a high integrity mold of precise dimensions. However, under certain conditions, the highly reactive molten metal poured into the mold may react chemically with the mold material and/or the metal oxide binder to produce a defective casting.
For example, if a zirconium dioxide binder is used in the fabrication of the mold, it may be reduced by molten titanium with the production of zirconium suboxides and oxygen. The latter reacts with the molten titanium, causing alpha phase stabilization and resulting in the production of a defective casting with an embrittled surface. Also, the zirconium suboxide products, some of which are gases at the casting temperature, have the potential of producing porosity in the casting.
It is the object of the present invention to provide a method which overcomes this problem and which produces a mold free from the above noted defects.